Electrochemical cells which utilize a negative electrode that is capable of reversibly, electrochemically storing hydrogen are known in the art. Such rechargeable electrochemical cells utilize a negative electrode which is formed of a hydrogen storage alloy, one example of such a negative electrode hydrogen storage alloy being a transition metal based alloy. The electrochemical cell also employs a positive electrode typically formed of nickel hydroxide material. Rechargeable electrochemical cells of this nature are commonly referred to as "nickel metal hydride cells" (NiMH) due to the nickel hydroxide positive electrode and the hydride nature of the metal used in the negative electrode.
When an electrical potential is applied between the electrolyte and the metal hydride negative electrode of a NiMH cell, the negative electrode (M) is charged by the electrochemical absorption of hydrogen and the electrochemical evolution of a hydroxyl ion: EQU M+H.sub.2 O+e.sub.-- .fwdarw.M-H+OH.sub.-- (Charge)
Upon discharge, the negative electrode material releases the stored hydrogen to form a water molecule and consumes an hydroxyl ion: EQU M-H+OH.sub.- .fwdarw.M+H.sub.2 O+e.sub.-- (Discharge)
The reactions are reversible.
When an electrical potential is applied between the electrolyte and the nickel hydroxide positive electrode, the electrode releases hydrogen to form a water molecule and consumes one hydroxyl ion: EQU Ni(OH).sub.2 +OH.sub.-- .fwdarw.NiOOH+H.sub.2 O+e.sub.-- (Charge)
Discharge of the positive electrode is characterized by the electrochemical evolution of a hydroxyl ion: EQU NiOOH+H.sub.2 O+e.sub.-- .fwdarw.Ni(OH).sub.2 +OH.sub.-- (Discharge)
These reactions are also reversible.
A cell utilizing an electrochemically rechargeable hydrogen storage negative electrode offers important advantages over conventional secondary batteries. Rechargeable hydrogen storage negative electrodes offer significantly higher specific charge capacities (ampere hours per unit mass and ampere hours per unit volume) than do either lead negative electrodes of lead acid batteries or cadmium negative electrodes of NiCd batteries. As a result of the higher specific charge capacities, a higher density (in watt hours per unit mass or watt hours per unit volume) is possible with NiMH storage batteries than with the prior art systems, making NiMH storage cells particularly suitable for many commercial applications.
High density energy storage is achieved by providing disordered materials which can be tailor-made to greatly increase the reversible hydrogen storage characteristics required for efficient and economical battery applications. A battery which uses disordered materials as a hydrogen storage negative electrode is disclosed in commonly assigned U.S. Pat. No. 4,623,597 to Sapru, et al for RECHARGEABLE BATTERY AND ELECTRODE USED THEREIN, incorporated herein by reference. Disordered materials are designed to have a substantially increased density of storage and catalytically active sites which provide a significant improvement of hydrogen adsorption and desorption. Disordered materials are designed to have unusual electronic configurations, which result from varying three-dimensional interactions of constituent atoms and their various orbitals. The disorder comes from compositional and translational relationships of atoms that are not limited by crystalline symmetry in their freedom to interact.
In commonly assigned U.S. Pat. No. 4,551,400 to Sapru, Hong, Fetcenko and Venkatesan for HYDROGEN STORAGE MATERIALS AND METHODS OF SIZING AND PREPARING THE SAME FOR ELECTROCHEMICAL APPLICATION, incorporated herein by reference, a hydriding-dehydriding comminution process was first disclosed for initial size reduction of bulk ingots of hydrogen storage alloy material. The '400 patent teaches a comminution process where hydrogenation is conducted at a hydrogen gas pressure between about 100 and 2000 psi. A hydrogen gas pressure above about 200 psi. was necessary to form a hydride at room temperature in Sapru, et al. These conditions depend on the composition of the material and its geometry.
Suitable active materials for the negative electrode are also disclosed in the '400 patent. The materials described therein store hydrogen by reversibly forming hydrides. The materials of Sapru, et al have compositions of: EQU (TiV.sub.2-x Ni.sub.x).sub.1-y M.sub.y
where 0.2.ltoreq..times..ltoreq.1.0, 0.ltoreq.y.ltoreq.0.2 and M=Al or Zr; EQU Ti.sub.2-x Zr.sub.x V.sub.4-y Ni.sub.y
where 0.2.ltoreq..times..ltoreq.1.5, 0.6.ltoreq.y.ltoreq.3.5; and EQU Ti.sub.1-x Cr.sub.x V.sub.2-y Ni.sub.y
where 0.ltoreq..times..ltoreq.0.75, 0.2.ltoreq.y.ltoreq.1.0.
Reference may be made to U.S. Pat. No. 4,551,400 for further descriptions of these materials and for methods of making them.
Selected elements can be utilized to further modify the disorder by their interaction with these orbitals so as to create the desired local chemical environments.
Other suitable materials for the negative electrode are disclosed in commonly assigned U.S. Pat. No. 4,728,586, incorporated herein by reference, to Venkatesan, Reichman and Fetcenko for ENHANCED CHARGE RETENTION ELECTROCHEMICAL HYDROGEN STORAGE ALLOYS AND AN ENHANCED CHARGE RETENTION ELECTROCHEMICAL CELL. As described in the above referenced '586 patent of Venkatesan, et al, one class of particularly desirable hydrogen storage alloys comprises titanium, vanadium, zirconium, and nickel, and at least one modifier element selected from the group consisting of copper, iron manganese, cobalt, and chromium. The preferred alloys described in Venkatesan, et al are alloys of titanium, vanadium, zirconium, nickel, and chromium, especially alloys having a composition represented by the formula: EQU (Ti.sub.2-x Zr.sub.x V.sub.4-y Ni.sub.y)Cr.sub.z
where x is between 0.0 and 1.5, y is between 0.6 and 3.5, and z is an effective amount less than 0.20.
Commonly assigned U.S. Pat. No. 5,536,591 to Fetcenko, et al for ELECTROCHEMICAL HYDROGEN STORAGE ALLOYS FOR NICKEL METAL HYDRIDE BATTERIES, incorporated herein by reference, discloses an improved hydrogen storage alloy having a significant increase in the frequency of occurrence of nickel regions resulting in significantly increased catalysis and conductivity. The '591 patent discloses two methods of increasing the frequency of occurrence of enriched nickel regions. One method specifically formulates an alloy having a surface region that is etched to remove oxides leaving the enriched nickel regions. The second method includes formulating a first electrochemical hydrogen storage alloy and alloying a secondary alloy formulated to contain components that are corroded during etching to leave enriched nickel regions. The preferred alloy disclosed in the '591 patent is (Base alloy).sub.a Co.sub.b Mn.sub.c Fe.sub.d Sn.sub.e, where base alloy comprises 0.1 to 60 atomic percent V, 0.1 to 57 atomic percent Ni, and 0 to 56 atomic percent Cr; b is 0 to 7.5 atomic percent; c is 13 to 17 atomic percent; d is 0 to 3.5 atomic percent, e is 0 to 1.5 atomic percent; and a+b+c+d+e=100 atomic percent.
Subtle changes in the local chemical and structural order for the addition of modifiers have significant effects on the electrochemical properties of negative electrodes incorporating hydrogen storage alloys. A modified, multicomponent, multiphase, reversible electrochemical hydrogen storage alloy is disclosed in commonly assigned U.S. Pat. No. 5,096,667 to Fetcenko for CATALYTIC HYDROGEN STORAGE ELECTRODE MATERIALS FOR USE IN ELECTROCHEMICAL CELLS AND ELECTROCHEMICAL CELLS INCORPORATING THE MATERIALS, incorporated herein by reference. The invention disclosed in the '667 patent teaches the subtle changes in stoichiometry, by the addition of individual metallic substitutes in the Ti--V--Zr--Ni type structure, provides enhanced properties. According to the invention, it is possible to do one or more of the following: increase cycle life; increase the specific capacity; increase the mid-point voltage at various discharge rates; decrease the polarization at various discharge rates; increase the low temperature specific capacity; increase the low temperature mid-point voltage; decrease the low temperature polarization; or decrease the self-discharge rate.
The hydrogen storage alloy is initially formed as a bulk ingot from a melt. A method of producing a hydrogen storage alloy is disclosed in commonly assigned U.S. Pat. No. 4,948,423 to Fetcenko, Sumner, and LaRocca for ALLOY PREPARATION OF HYDROGEN STORAGE MATERIALS, incorporated herein by reference. The hydrogen storage negative electrodes utilizing the aforementioned alloys are of relatively high hardness. Indeed, these alloys can typically exhibit Rockwell "C" ("Rc") hardness of 45 to 60 or more. Moreover, in order to attain the high surface areas per unit volume and per unit mass necessary for high capacity electrochemical performance, the alloy must be in the form of fine particles. In a preferred exemplification, the hydrogen storage alloy powder must pass through a 200 U.S. mesh screen, thus being smaller than 75 microns in size (200 U.S. mesh screen has interstices of about 75 microns). Therefore, the resulting hydrogen storage alloy material is comminuted, e.g., crushed, ground, milled or the like, before the hydrogen storage material is fabricated into electrode form.
Comminution of bulk ingots of hydrogen storage alloy material is made more difficult because the materials described hereinabove are quite hard, and therefore do not easily fracture into particles of uniform size and shape. In commonly assigned U.S. Pat. No. 4,893,756 to Fetcenko, Kaatz, Sumner, and LaRocca for HYDRIDE REACTOR APPARATUS FOR HYDROGEN COMMINUTION OF METAL HYDRIDE HYDROGEN STORAGE MATERIAL, the disclosure of which is incorporated herein by reference, a hydride-dehydride cycle comminution process was disclosed for initial size reduction of bulk ingots of hydrogen storage alloy material to flakes of about 80-100 mesh size. While this process is effective for the initial size reduction of hydrogen storage alloy, it is inadequate for the task of further comminuting particulate hydrogen storage alloy powder to the required particle size of 75 microns or less (i.e. 200 mesh or less). The approximately 200-400 mesh size distribution has been experimentally determined to provide performance superior to other sizes of material in the negative electrode of metal hydride, hydrogen storage electrochemical cells.
Any method which can accomplish the objective of providing economical size reduction of the metal hydride material is a potential candidate for commercial processing. However, there are numerous characteristics of the material which require special handling, instrumentation and other precautions. These characteristics include: (1) inherent alloy powder hardness, i.e., approximately Rockwell "C" ("Rc") 60 hardness. This means that conventional size reduction processes of shear, abrasion and some types of impact mechanisms as ball mills, hammer mills, shredders, fluid energy, and disk attrition, are not very effective; (2) sensitivity to oxidation, such that comminution must be done under an inert environment to provide a safe environment and maintain acceptable electrochemical performance; (3) requirement of a specific crystalline structure necessary for electrochemical activity; i.e., the microstructure of the material cannot be adversely altered during grinding or atomization to produce powders directly from a melt; and (4) requirement of a broad particle size distribution with a maximum size of 75 microns (200 mesh) which provides optimum packing density and electrochemical accessibility.
Early attempts to provide a method for size reduction of hydrogen storage alloy materials proved inadequate due to the extreme hardness of the hydrogen storage alloy materials. Conventional size reduction techniques employing devices such as jaw crushers, mechanical attritors, ball mills, and fluid energy mills consistently fail to economically reduce the size of such hydrogen storage materials. Grinding and crushing techniques have also proven inadequate for initial reduction of ingots of hydrogen storage alloy material to intermediate sized (i.e. 10-100 mesh) particulate
There are numerous methods for preparing metal powders. Since the alloys under consideration are at one stage molten, one might consider ultrasonic agitation or centrifugal atomization of the liquid stream to prepare powders directly. The cost and the product yield are the two main concerns with using this approach. The particle shape is also not optimal. Finally, because it is difficult to provide a completely inert atmosphere; surface layers, which are undesirable from an electrochemical perspective, may be formed on the particulate.
Attempts to embrittle the hydrogen storage alloy material by methods such as immersion in liquid nitrogen, so as to facilitate size reduction are inadequate because: (1) the materials are not sufficiently embrittled; (2) the methods typically introduce embrittlement agents in the alloys which have an undesirable effect upon the electrochemical properties of the hydrogen storage alloy material; and (3) as the materials become more brittle, it becomes increasingly difficult to obtain uniform particle size distribution. Other methods for embrittling metals are disclosed, for example, in Canadian patent No. 533,208 to Brown. However, Brown identifies many disadvantages of treating vanadium metal (a component of the metal hydride storage material) with hydrogen gas. Brown employs cathodic charging as a size reduction technique.
Hydrogen storage materials are not amenable to mechanical size reduction techniques due to their inherent high material hardness. Frequently, significant wear of the grinding medium is observed. Even if one decides to use such mechanical techniques, it is difficult to attain the desired range of particle size since the materials are difficult to grind. A high fraction of powder just under the specified maximum value is frequently observed. Also, grinding processes invariably have low yield factors, that is, an unacceptably high fraction of the feed stock of particulate hydrogen storage alloy is oversized. In commonly assigned U.S. Pat. No. 4,915,898 to Wolff, Nuss, Fetcenko, Lijoi, Sumner, LaRocca, and Kaatz for METHOD FOR THE CONTINUOUS FABRICATION OF COMMINUTED HYDROGEN STORAGE ALLOY MATERIAL NEGATIVE ELECTRODES, MATERIAL an improved comminution process for metal hydride particulate production was achieved by a hydride-dehydride cycle followed by an impact cycle. While this process is an excellent particle size reduction process, the process remains partially mechanical in nature in that final particle size reduction is achieved by propelling the particulate material against an impact block, therefore requiring the rotor chamber to be cleaned between batches. Furthermore, component part life is short because of the hardness of the hydrogen storage material. Obviously, any processing for size reduction after hydriding adds additional complexity and cost, as a result of additional handling, material transfer, capital equipment, process energy and gasses.
Accordingly, there exists a need in the art for an improved method of forming a powder of a hydrogen storage alloy without the need for mechanical processing following the hydriding/dehydriding step.